The present application relates generally to the field of retractors for use in occupant restraint systems. More specifically, this application relates to progressive load limiting devices for use in seatbelt systems to restrain occupants.
Seatbelt systems for vehicles typically include an energy management feature to absorb or reduce the kinetic energy of the seat belt system. For example, the seat belt systems for the front seats of a vehicle typically include seat belt retractors with devices to absorb or reduce the kinetic energy of a seat belt during an event, such as a collision in which the bodily weight of a seat occupant may be applied to the seat belt as the vehicle is suddenly decelerated. By reducing the kinetic energy of the seat belt, such as by slowing an unwinding rate of the seat belt and/or reducing a force applied to the seat belt may provide improved performance.
For example, conventional seat belt retractors typically utilize a torsion bar to absorb or reduce the kinetic energy of the seat belt retractor and thus the seat belt. One type of retractor which uses a torsion bar is a retractor with a single torsion bar, which reduces kinetic energy to a certain level. Another type of retractor is a digressive system which initially reduces kinetic energy by a relatively large amount but then switches so that kinetic energy is reduced by a relatively smaller amount after a certain number of rotations of the spool of the retractor.
The digressive retractor system is particularly useful for large occupants who can initially exert a large amount of force to a seat belt during an event, such as during a collision when a vehicle is suddenly decelerated. However, such digressive systems do not provide the best performance for smaller sized vehicle occupants, who would exert relatively smaller forces on a seat belt during an event.
One possible solution is to use a seat belt retractor with a torsion bar which reduces the kinetic energy of the retractor and seat belt by a relatively small amount which corresponds to the amount of energy produced by a small occupant. However, although such a design may provide good results when tested for small occupants, such a design might not provide good results when tested for relatively large occupants due to the relatively large forces and relatively large amounts of kinetic energy that such large occupants can produce.